This invention relates to a transmission system for color television signals having improved signal-to-noise (SN). In magnetic recording of television signals by video tape recorders, the record or playback head may occasionally fail to come into contact with the magnetic oxide coating because of variations in tape tension, build up of dirt on the heads and the like. This may cause a reduction in the amplitude of the signal transduced from the tape to the playback head, and may result in objectionable distortion. It is known to frequency-modulate a carrier with the video information to be recorded. Such frequency modulation of the signal translates amplitude changes of the video signal into frequency changes of the carrier. An amplitude limiter coupled to receive the frequency-modulated signal strips away amplitude variations resulting from imperfect head contact, and the frequency-modulated signal when demodulated has an improved signal-to-noise compared with the case of direct recording.
When color television signals encoded in a standard NTSC manner including luminance components and chrominance components quadrature-modulated onto a color subcarrier are recorded, the total frequency bandwidth of the vieo signal is large. When it is desired to record such an NTSC signal, it is found that that total bandwidth of the NTSC signal is so large that the sidebands of the frequency-modulated carrier extend over a greater frequency band than can be encompassed within the FM channel of the recorder. Consequently, the "color under" system has been used in the past. In this system, the color subcarrier, quadrature modulated with chroma components, is directly recorded at a low frequency on the same tape track with an FM carrier modulated by video luminance information. To improve linearity, the directly recorded chrominance information is recorded with the aid of a bias signal. To prevent interaction between the bias signal and the frequency-modulated carrier, the FM carrier is often used as the bias signal. While such an arrangement allows recording of a color television signal on a single track of a video tape recorder, certain problems exist, such as poor SN of the chrominance signal, crosstalk between the two quadrature-modulated color signals, and limited freuency bandwidth which necessitates reduction of the desired bandwidth in either the chrominance or luminance information, or possibly both. Furthermore, the FM luminance carrier cannot be modulated to the maximum possible amount because maximum modulation drives the recording medium into saturation, adding distortion to the directly recorded chrominance information.
In order to improve the quality of the television signal to broadcast standards, the luminance information may be recorded on a first track of the tape by the use of a frequency-modulated carrier, while at the same time recording the quadrature-modulated chrominance information onto a second track of the tape adjacent the first. The chrominance information is modulated onto a frequency-modulated carrier for improved signal-to-noise. It has been found, however, that broadcast quality may not be achieved even in such a system using two wideband channels for the recording of the video information. Furthermore, it has been found that cross-modulation occurs between the two components of the chrominance signal.
FIG. 8 illustrates an amplitude-frequency diagram. In FIG. 8, a frequency F0 illustates the rest frequency of a frequency-modulated carrier. FLO and FHI represent the lower and upper deviation frequency limits, respectively. An envelope 810 illustrates the amplitude-frequency characteristic of a transmission channel generally including, for example, a tape recorder channel. At frequencies F14 and F16 the response of the channel is reduced due to filters, inherent frequency limitations and the like. A series of spectral lines 812 illustrates generally the distribution of energy which results from modulating the carrier with a relatively low-frequency video signal. Many spectral lines appear, the amplitude of which depends upon the amplitude of the modulating signal. FIG. 9 illustrates the response of the same system modulated with a relatively high-frequency video signal. Very few spectral lines 822 appear within the passband defined by curve 810. Other spectral lines illustrated as 824 are cut off and do not appear. It has been established that the signal-to-noise ratio of a frequency-modulated transmission channel such as that described is degraded at higher modulating frequencies. This may be explained by the loss of many of the spectral lines associated with the information of the signal in the case of the high-frequency signal, as compared with the low-frequency situation in which large numbers of spectral lines are carried through the channel.
In order to obtain improved characteristics for a two-channel tape recorder or other transmission system, it is desirable to reduce the frequency of the signal modulating the chrominance channel. Comprison of the bandwidth of the baseband I and Q signals of FIGS. 3a and 3b with the bandwidth of the I and Q signals quadrature-modulated onto a subcarrier, as illustrated in FIG. 3f, reveals that each of the baseband signals alone has a lesser bandwidth than does the modulated signal. The frequency bandwidth of the signal modulating the chrominance channel may be reduced by alternately modulating the frequency-modulated carrier in the chrominance signal channel with one of the two chrominance signals representing the chrominance information. For example, if the chrominance information is represented by I and Q signals, where the I signal has a frequency bandwidth of 1 MHz and the Q signal has a frequency bandwidth of 0.5 MHz, each of these signals is alternately modulated onto the carrier for coupling into the channel. Alternation, however, results in a loss of I signals during that interval in which Q signals are being carried through the system, and similarly Q signals are lost during that interval in which I signals are being processed. Thus, there is a loss in signals similar to that which occurs in a SECAM system. In the SECAM system, the line-to-line alternation of the chrominance information results in a reduced vertical chrominance resolution which degrades the picture. U.S. Pat. No. 4,163,248 issued July 31, 1979 to Heitmann describes a system for alternately processing luminance and chrominance information through a digital field store; the loss of information is concealed by repetition of the stored chrominance information during display of unstored luminance and repetition of stored luminance information during display of unstored chrominance. A television transmission system, which may include a record, and having high signal-to-noise, low cross-modulation and high resolution (no information loss) in the chrominance channel is desirable.